In the past, automobiles employed a so-called frame construction in which parts such as an engine, a radiator, a suspension, a transmission, a differential, a fuel tank, and the like were mounted on a frame formed by assembling members with a box-shaped cross section in the form of a ladder, and then mounting a body having an engine compartment, a passenger compartment, and a trunk atop the body. However, a frame construction always uses a heavy frame which is a separate member from the body, so it is difficult to decrease the weight of the body. In addition, since a process of joining the frame to the body is unavoidable, productivity is poor. Therefore, almost all automobiles manufactured in recent years have a monocoque body (unit construction body) in which the frame and the body are integral with each other.
A monocoque body supports a load atop an integral body shell comprising a body side formed by combining a side sill, an A-pillar, a B-pillar, a roof rail side member, and in some cases a C-pillar with an underbody (also referred to as a platform) which is the most important part and forms the base of the body structure and is the bottom surface, namely the floor portion of a monocoque body. When portions of the body contract or collapse under an externally applied impact load, the impact energy is absorbed by the body parts as a whole.
A monocoque body does not have a clearly defined frame as is the case with a frame construction, but in portions where loads and stress concentrate such as mounting portions for the engine and a suspension, the body shell is reinforced by suitable installation of automobile body strength members formed from tubular members with a closed cross section such as side members, suspension members, various pillars, cross members, roof rail side members, and side sills. The body side and the underbody not only greatly affect the bending stiffness and torsional stiffness of an automobile body, but at the time of a side impact, they have the function of minimizing damage to the passenger compartment and increasing the safety of passengers. In particular, compared to a front impact, it is difficult to adequately guarantee space for protecting passengers during a side impact, so it is important to increase the stiffness of the body side.
Among strength members which are disposed in this manner are “side members” (also referred to as the subframe). These members form the skeleton which is interposed when mounting the suspension, the engine, the transmission, or the like on the underbody. The underbody greatly affects the various types of stiffness (such as the bending stiffness and the torsional stiffness) of the body for supporting the suspension and the drive train, so by suitably installing side members and other reinforcing members in various portions of the underbody, the underbody is given sufficient stiffness. One such side member is a front side member which extends generally horizontally in the fore and aft direction on the left and right sides of the engine compartment and is welded in place.
Normally, a front side member has a body comprising a tube having a closed cross section having a shape such as a rectangle, a hexagon, a circle, or the like. The body has a front end portion which extends in the axial direction of the body from one end of the body towards the other end of the body in the fore and aft direction of the vehicle body, a sloping portion which is continuous with the front end portion and which is sloped along the dash panel which is a wall between the engine compartment and the passenger compartment, and a rear end portion which is continuous with the sloping portion and extends along the floor panel is connected to the dash panel. Although it depends upon the size of the vehicle body, the overall length of the front side member is around 600-1200 mm.
As stated above, a front side member is a strength member, the most important requirement of which is to maintain the strength of the underbody. Therefore, it is designed so as to have adequate strength. It is also the main member which bears an impact load applied at the time of a front impact collision. Accordingly, it is designed so that if a front impact collision occurs, it has impact absorbing properties such that it can absorb impact energy by plastic deformation of its front end by buckling into an accordion shape. In this manner, a front side member must have the mutually opposite properties that it have adequate strength and that its front end portion easily undergo plastic deformation into the shape of an accordion when an impact load is applied.
As stated above, a front side member is welded to other panels as a reinforcing member for the underbody, so it is also required to have excellent weldability and excellent workability such that it can have a complicated shape from its front end portion to its rear end portion and such that it can be subjected to punching or cutting.
Patent Document 1 discloses an invention pertaining to an energy absorbing member which comprises a hollow aluminum alloy extrusion having a plate thickness which locally varies. Patent Document 2 discloses an invention pertaining to a front side member which has a closed cross section with an arch-shaped portion disposed parallel to the fore and aft direction of a vehicle body and which has a plate thickness which locally varies. Patent Document 3 discloses an invention pertaining to a front side member having a weak portion provided in its front end portion. Patent Document 4 discloses an invention pertaining to a front side member in which the shape of its front end portion is such that it can more uniformly deform by buckling over its entire cross section. Patent Document 5 discloses an invention pertaining to a front side member having a closed cross section and comprising a lower member with a U-shaped cross section comprising a casting of a light alloy and an upper member comprising a plate of a light alloy.
Patent Document 6 discloses an invention which prevents buckling of the A-pillar at the time of rollover by installing a reinforcing tube inside the A-pillar for the body side.
In recent years, there has been an increasing demand for decreases in weight and increases in strength of strength members for automobile bodies in order to increase fuel efficiency so as to decrease discharge of CO2 in order to suppress global warming as well as to increase the safety of passengers at the time of a collision. In order to cope with such demands, high strength materials such as high tensile strength steel plates having a tensile strength of at least 780 MPa or even at least 900 MPa which is considerably higher than conventional strength levels are now much used.
At the same time that such materials are being increased in strength, the structure of strength members for automobile bodies is being reconsidered. For example, in order to enable application to various automobile parts, there is a strong demand for the development of bending techniques which can work strength members for automobile bodies having a widely varying bent shape such as those which are manufactured by bending with a bending direction which varies 2-dimensionally such as S-bending or bending with a bending direction which varies three-dimensionally with high accuracy.
Various working techniques have been proposed in order to cope with such demands. For example, Patent Document 7 discloses an invention pertaining to a method of bending while performing heat treatment of a metal pipe or the like by gripping the end portion of a material being worked such as a metal pipe with a rotatable arm, and while heating with a heating device, gradually moving the heated portion in the axial direction to produce bending deformation and then immediately thereafter performing cooling. Patent Document 8 discloses an invention pertaining to a method of bending while performing heat treatment of a metal pipe or the like by gripping a metal pipe and applying a twisting force and a bending force to a heated portion carry out bending deformation while twisting the metal pipe.
Taking into consideration decreases in the weight of products Mimed by bending (referred to below as bent products), the tensile strength of the products is preferably set to be at least 900 MPa and more preferably at least 1300 MPa. Up to now, in order to achieve such a strength, as disclosed in Patent Documents 7 and 8, a pipe having a tensile strength of 500-700 MPa was used as a starting material and subjected to bending, after which its strength was increased by heat treatment to manufacture a bent product having a desired high strength.
The inventions disclosed in Patent Documents 7 and 8 both use working method classified as so-called grip bending. In order to carry out either invention, it is necessary to grip the end of a material being worked with a rotatable arm. Furthermore, each time the material being worked is regripped by the arm, it is necessary to return the aim to its original position, so the feed speed of the material being worked greatly varies, it becomes difficult to perform complicated control of the cooling rate, and a desired quenching accuracy cannot be obtained. Therefore, the speed of heating and cooling must be controlled in a complicated manner and which high accuracy in order to produce non-uniform strains, and it is extremely difficult to obtain a desired quenching accuracy. Therefore, variations in the bent shape develop, and particularly in the case of high strength materials, delayed fracture caused by residual stresses develop, and it is difficult to manufacture a strength member for automobiles requiring high reliability.
Patent Document 9 discloses an invention pertaining to a bending apparatus with high frequency heating in which a material to be worked which is supported by a support means is fed from an upstream side towards a downstream side by a feed device while bending is carried out downstream of the support means, and a roller is supported so as to move three-dimensionally. According to the bending apparatus with high frequency heating disclosed in Patent Document 9, the roller straddles the material being worked and moves to opposite side surfaces of the material being worked, contacts the side surfaces, and performs bending. Therefore, even when bending is carried out in which the bending direction varies two-dimensionally such as with S-bending, it is no longer necessary to perform a tooling operation of rotating the material being worked by 180 degrees, so working can be efficiently carried out.
However, the bending apparatus with high frequency heating disclosed in Patent Document 9 does not have any means for clamping the material being worked on both sides. Therefore, deformation caused by residual stress due to cooling after high frequency heating easily develops, which makes it difficult to obtain a desired dimensional accuracy In addition, the working speed is limited, and it is difficult to increase the degree of bending.
Patent Document 10 discloses an invention pertaining to a bending apparatus which in place of the above-described gripping working or roller of a bending apparatus with high frequency heating provides a fixed die installed in a fixed position and a movable gyro-die which is spaced from the fixed die and can move three-dimensionally. A heating means heats a metal material to a temperature corresponding to the bending curvature of a metal material by the movable gyro-die.    Patent Document 1: JP 10-45023 A    Patent Document 2: JP 11-255146 A    Patent Document 3: JP 2001-106002 A    Patent Document 4: JP 2002-173055 A    Patent Document 5: JP 2003-306171 A    Patent Document 6: JP 2003-118633 A    Patent Document 7: JP 50-59263 A    Patent Document 8: Japanese Patent No. 2816000    Patent Document 9: JP 2000-158048 A    Patent Document 10: Japanese Patent No. 3195083